1. Field of the Invention
The present invention relates to a zoom lens suitable for an image pickup apparatus such as a video camera, a still film/digital camera and a TV camera.
2. Related Background Art
Image pickup apparatuses such as video cameras and digital still cameras require, as an image taking optical system, a zoom lens having a wide angle of view, a large aperture ratio and high optical performance. Further, the image pickup apparatuses require a relatively long back focus since various optical members such as a low-pass filter and a color correction filter are disposed between a rearmost lens and an image plane. In addition, the image pickup apparatuses require a good image side telecentric characteristic to avoid color shading.
A negative lead type zoom lens in which a lens unit having a negative refractive power is disposed closest to an object is known as a small zoom lens having a long back focus and a good image side telecentric characteristic.
Japanese Patent Laid-Open Nos. 2004-061675 and 2004-094283 disclose zoom lenses each being constituted by three lens units including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a positive refractive power in order from an object side to an image side (image plane side). U.S. Pat. No. 7,019,911 and Japanese Patent Laid-Open No. 2006-65034 disclose zoom lenses with a three lens unit configuration and a high zoom ratio in which all lens units are moved for zooming. U.S. Pat. No. 4,838,666 and Japanese Patent Laid-Open No. 07-261083 disclose zoom lenses with a three lens unit configuration, a wide angle of view, a long back focus and a good image side telecentric characteristic.
Moreover, it is known that correcting distortion of a zoom lens by electrical image processing decreases the distortion which is increased by widening its angle of view.
Furthermore, in order to miniaturize a camera and obtain a high zoom ratio, a retractable zoom lens is known whose length in an optical axis direction is reduced by shortening distances between lens units in a non-used state as compared with those in a used state. In such a retractable zoom lens, a lot of number of the lens units constituting the zoom lens makes the length thereof long, which makes it impossible to reduce the length of the retracted zoom lens.
It is important for achieving miniaturization of the entire zoom lens, reduction of the length of the retracted zoom lens, a high zoom ratio and high optical performance to appropriately set a configuration of each lens unit and an image-forming magnification of the zoom lens.